The present disclosure relates generally to electric double layer capacitors and more specifically to carbon-based composite electrodes comprising microporous activated carbon for use in such capacitors.
Energy storage devices such as ultracapacitors may be used in many applications such as where a discrete power pulse is required. Example applications range from cell phones to hybrid vehicles. Energy storage devices typically comprise a porous separator and/or an organic electrolyte sandwiched between a pair of carbon-based electrodes. The energy storage is achieved by separating and storing electrical charge in the electrochemical double layer at the interfaces between the electrolyte and the electrodes. Important characteristics of these devices are the energy density and power density that they can provide, which are both largely determined by the properties of the carbon-based electrodes.
Carbon-based electrodes suitable for incorporation into high energy density devices are known. The carbon materials, which form the basis of such electrodes, can be made from natural or synthetic precursor materials. Known natural precursor materials include coals, nut shells, and biomass, while synthetic precursor materials typically include phenolic resins. With both natural and synthetic precursors, carbon materials can be formed by carbonizing the precursor and then activating the resulting carbon. The activation can comprise physical (e.g., steam) or chemical activation.
A property of the carbon that can influence its success when incorporated into high energy density devices such as electric double layer capacitors (EDLCs) is the material's specific capacitance. Higher specific capacitance generally results in a higher volumetric energy density of the resulting device. Accordingly, it would be an advantage to provide activated carbon materials as well as methods for making activated carbon materials having a high specific capacitance. Such materials can be used to form carbon-based electrodes that enable higher energy density devices.
According to one embodiment, an electric double layer capacitor electrode includes a current collector having opposing major surfaces, first and second conductive layers formed over respective ones of the major surfaces, and first and second carbon-based layers formed over the first and second conductive layers. The carbon-based layer comprises microporous activated carbon.
The activated carbon may be characterized by its pore size distribution, where pores having a size of ≦1 nm provide a combined pore volume of ≧0.3 cm3/g, pores having a size of from >1 nm to ≦2 nm provide a combined pore volume of ≧0.05 cm3/g, and the activated carbon comprises <0.15 cm3/g combined pore volume of any pores having a size of >2 nm.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description and the claims.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments and together with the description serve to explain the principles and operations of the invention.